1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for providing increased design flexibility for RF circuits, and more particularly for optimization of dielectric circuit board materials for improved performance in filters having two ports and coupled resonant lines.
2. Description of the Related Art
RF circuits, transmission lines and antenna elements are commonly manufactured on specially designed substrate boards. For the purposes of these types of circuits, it is important to maintain careful control over impedance characteristics. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. Electrical length of transmission lines and radiators in these circuits can also be a critical design factor.
Two critical factors affecting the performance of a substrate material are permittivity (sometimes called the relative permittivity or ∈r) and the loss tangent (sometimes referred to as the dissipation factor). The relative permittivity determines the speed of the signal, and therefore the electrical length of transmission lines and other components implemented on the substrate. The loss tangent characterizes the amount of loss that occurs for signals traversing the substrate material. Losses tend to increase with increases in frequency. Accordingly, low loss materials become even more important with increasing frequency, particularly when designing receiver front ends and low noise amplifier circuits.
Printed transmission lines, passive circuits and radiating elements used in RF circuits are typically formed in one of three ways. One configuration known as microstrip, places the signal line on a board surface and provides a second conductive layer, commonly referred to as a ground plane. A second type of configuration known as buried microstrip is similar except that the signal line is covered with a dielectric substrate material. In a third configuration known as stripline, the signal line is sandwiched between two electrically conductive (ground) planes. Ignoring loss, the characteristic impedance of a transmission line, such as stripline or microstrip, is equal to {square root over (Ll/Cl)} where Ll is the inductance per unit length and Cl is the capacitance per unit length. The values of Ll and Care generally determined by the physical geometry and spacing of the line structure as well as the permittivity of the dielectric material(s) used to separate the transmission line structures. Conventional substrate materials typically have a relative permeability of approximately 1.0.
In conventional RF design, a substrate material is selected that has a relative permittivity value suitable for the design. Once the substrate material is selected, the line characteristic impedance value is exclusively adjusted by controlling the line geometry and physical structure.
Radio frequency (RF) circuits are typically embodied in hybrid circuits in which a plurality of active and passive circuit components are mounted and connected together on a surface of an electrically insulating board substrate such as a ceramic substrate. The various components are generally interconnected by printed metallic conductors of copper, gold, or tantalum, for example that are transmission lines as stripline or microstrip or twin-line structures.
The permittivity of the chosen substrate material for a transmission line, passive RF device, or radiating element determines the physical wavelength of RF energy at a given frequency for that line structure. One problem encountered when designing microelectronic RF circuitry is the selection of a dielectric board substrate material that is optimized for all of the various passive components, radiating elements and transmission line circuits to be formed on the board. In particular, the geometry of certain circuit elements may be physically large or miniaturized due to the unique electrical or impedance characteristics required for such elements. For example, many circuit elements or tuned circuits may need to be an electrical xc2xc wave. Similarly, the line widths required for exceptionally high or low characteristic impedance values can, in many instances, be too narrow or too wide respectively for practical implementation for a given substrate. Since the physical size of the microstrip or stripline is inversely related to the relative permittivity of the dielectric material, the dimensions of a transmission line can be affected greatly by the choice of substrate board material.
Still, an optimal board substrate material design choice for some components may be inconsistent with the optimal board substrate material for other components, such as antenna elements or filters. Moreover, some design objectives for a circuit component may be inconsistent with one another. For example, it may be desirable to reduce the size of an antenna element. This could be accomplished by selecting a board material with a relatively high dielectric. However, the use of a dielectric with a higher relative permittivity will generally have the undesired effect of reducing the radiation efficiency of the antenna. Accordingly, the constraints of a circuit board substrate having selected relative dielectric properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit.
An inherent problem with the foregoing approach is that, at least with respect to the substrate material, the only control variable for line impedance is the relative permittivity, ∈r. This limitation highlights an important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely Ll, the inductance per unit length of the transmission line.
Yet another problem that is encountered in RF circuit design is the optimization of circuit components for operation on different RF frequency bands. Line impedances and lengths that are optimized for a first RF frequency band may provide inferior performance when used for other bands, either due to impedance variations and/or variations in electrical length. Such limitations can limit the effective operational frequency-range for a given RF system.
Conventional circuit board substrates are generally formed by processes such as casting or spray coating which generally result in uniform substrate physical properties, including the permittivity. Accordingly, conventional dielectric substrate arrangements for RF circuits have proven to be a limitation in designing circuits that are optimal in regards to both electrical and physical size characteristics.
In a first embodiment according to the invention, a circuit for processing radio frequency signals comprises a substrate where the circuit can be placed. The substrate includes at least one dielectric layer having a first set of substrate properties over a first region and at least a second set of substrate properties over a second region. The second set of substrate properties can be different than the first set of substrate properties. The circuit further comprises at least one ground plane coupled to the substrate and a interdigital filter having a plurality of resonator elements with at least a portion of alternating adjacent ends of the plurality of resonator elements coupled to the at least one ground plane and at least a portion of the interdigital filter coupled to the second region.
In a second embodiment according to the invention, a printed circuit for processing radio frequency signals comprises a substrate upon which the circuit can be placed. The substrate includes at least one dielectric layer having a first set of dielectric and magnetic properties over a first region and at least a second set of dielectric and magnetic properties over a second region. The second set of dielectric and magnetic properties provide a different dielectric permittivity and magnetic permeability as compared to the first set of dielectric properties. The printed circuit further comprises at least one ground disposed in or on the substrate and an interdigital filter. The interdigital filter includes a plurality of resonator elements with at least a portion of alternating adjacent ends of the plurality of resonator elements coupled to the at least one ground plane and at least a portion of the interdigital filter coupled to the second region.
In a third embodiment of the present invention, a printed circuit for processing radio frequency signals comprises a substrate including substrate regions upon which the circuit can be placed. The circuit is an interdigital filter including a plurality of resonator line elements and at least one ground coupled to said substrate. The plurality of resonator line elements are at least partially coupled to respective substrate regions that have substrate characteristics that are each independently customizable.